This invention relates to analyzers, and specifically to an electrochemical fuel cell analyzer for measuring concentrations of alcohol in a breath sample.
The current emphasis for detection of blood-alcohol content is aimed at the drinking driver which, it is estimated, accounts for approximately 50 percent of all traffic fatalities occurring in the United States. However, current methods for measuring the alcohol concentrations in an individual's blood have proved to be either cumbersome or inaccurate. As a result, law enforcement agencies and others have sought a device capable of measuring an individual's blood-alcohol concentration by detecting alcohol concentrations in alveolar or deep lung air samples. The concentration of alcohol molecules in such breath samples has been found to be directly proportional to the concentration of alcohol in the individual's blood.
Currently available breath-alcohol instruments employ chemical reagents which interact with alcohol to determine the alcohol level by a colorimetric method. This type of analyzer, however, is relatively inaccurate, expensive and bulky as well as requiring an alternating current source and skilled personnel for its operation. Because of the lack of a portable quantitative instrument, the mobile law enforcement officer frequently does not make an arrest unless he first conducts a time consuming "field sobriety test" of coordination and reactions or the driver is unmistakably intoxicated. As a result, the majority of drivers whose motor functions may be severely impaired by alcohol are permitted to remain on the road at considerable risk to themselves and others.
Electrochemical detectors have been considered but to date investigation has been restricted to ordinary polarographic systems employing a cell with an anode, a counterelectrode and a reference electrode. The sensing anode in these systems is maintained at a predetermined voltage with respect to the reference electrode. An operational amplifier senses the potential difference between the reference and sensing electrodes to generate an error signal that determines the current flow between the counterelectrode and sensing electrode. This potential difference varies according to the rate of alcohol oxidation at the anode so that the current between electrodes changes in proportion to the alcohol concentration at the sensing electrode. However, a significant background current is always present in such systems, even in the absence of alcohol, frequently with magnitudes approaching the currents resulting from alcohol oxidation. Such background currents tend to be unstable due to changes in electrolyte concentration and electrode surface conditions, causing serious signal drift problems and other inaccuracies. In addition, the electrodes may become slowly deactivated by formation of oxides on their surfaces, and the reference electrode is subject to a continual degradation creating further long-term signal drift.
In contrast, the fuel cell breath-alcohol analyzer of this invention provides a compact, inexpensive and portable instrument that can be battery operated for field use. The cell itself operates without applying external electrical power because alcohol provides the fuel for generating the electrical signal current. Since no appreciable current flows through the cell in the absence of alcohol and no voltage difference need be maintained during cell operation, there is little, if any, background current, and the associated drift problems and other inaccuracies due to oxide formation and electrode degradation are virtually eliminated. Because the cell components are not consumed in the reaction, the cell has a practically unlimited life and is correspondingly simple and inexpensive to construct, operate and maintain. Furthermore, by operating in the alcohol diffusion limited mode, the short circuit current of the cell becomes linearly proportional to the alcohol concentration in an alveolar breath sample that corresponds directly with the individual's blood-alcohol concentration.